docs: def apply(self, df):
          """Compute aggregate over DataFrame"""
  
          if self.subset:
              if _axis_is_rows(self.axis):
                  df = df[self.subset]
              if _axis_is_cols(self.axis):
                  df = df.loc[self.subset]
  
          result = df.agg(self.func, axis=self.axis, *self.args, **self.kwargs)
          result.name = self.title
          return result